What's alignment? Does it mean anything in 5e?

My first exposure to the D&D alignment system was in old DOS games where you could pick your alignments from what looked to me like a jumble of words like "good" and "chaotic." I didn't understand what any of the terms meant, and they all looked like "chaotic true neutral good" to me. Now, of course, I understand that an alignment like that is impossible on the good-evil, lawful-chaotic axes, and I promise I'm rolling my eyes at my younger self just as hard as you are. Hillsfar was my favorite of the few old SSI games we had, because it was basically a burglary simulator. Yes, it also had an archery simulator and an arena simulator and a brute-force-password-cracker simulator (because your older siblings lost the copy-protection code wheel years ago), but the best things to do involved breaking into buildings and looting them. Even if you liked fighting in the arena, you could loot buildings until you were caught and thrown in the arena. T...